Optical coherence tomography (OCT) is an optical technique for acquiring three dimensional structural information about a sample. A broadband light source emits light that is split into a reference light beam and a sample light beam using a beam splitter. The sample light beam is incident upon a sample, which reflects a portion of the sample light beam back towards the beam splitter. The reference light beam is incident upon a reference mirror, which reflects the reference light beam back towards the beam splitter. The reflected sample light beam and the reflected reference light beam are then mixed at the beam splitter, if the optical path difference between the reference light beam and sample light beam is within the coherence length of the light source then it results in interference of the two beams of light. The resulting interference signal is detected by a single detector in time domain optical coherence tomography (TD-OCT) and by spectrometer in Fourier domain OCT which is another variant of OCT. Information about the structure of the sample is obtained by analyzing the interference pattern. TD-OCT requires both axial and lateral scans of the sample to obtain a three-dimensional image. The axial scan allows information about the depth profile of a sample to be obtained by adjusting the position of the reference mirror, while the lateral scan allows information about the lateral profile of the sample to be obtained by raster scanning the probe light beam or the sample itself. In frequency domain OCT the reference mirror is kept fixed and the depth information of the sample is obtained by Fourier transforming the spectrometer data which gives axial scan of the sample in a single shot. A 3D profile of the sample is then acquired by scanning the sample or the sample beam itself.
Conventional OCT techniques use light with a frequency in the near infrared and/or optical range. However, these frequencies of light have low penetration depth and high attenuation and scattering in certain materials. Terahertz (THz) radiation, on the other hand, have large penetration depth, higher transmission, and low scattering in several materials where NIR and visible radiation does not. Recently, time domain coherence tomography (TD-CT) of a sample using THz radiation was demonstrated where THz radiation were detected using conventional electo-optic (EO) sampling. In the EO sampling technique, a probe light beam of optical or infrared frequency is mixed with the THz radiation in a nonlinear material. The presence of the THz radiation in the nonlinear material at the same time as the probe light results in the rotation of the polarization of the probe light. A polarizer after the nonlinear material followed by a photodetector operating at the frequency of the probe light therefore allows the intensity of the THz radiation to be measured. A delay stage in the path of the probe light is used to scan the time delay between the probe light and the THz radiation. Making measurements at different time delay values allows the entire THz radiation waveform to be mapped in time.